In a plant control system controlling, for example, a steelmaking plant, a LAN is often constructed with a process computer managing and controlling the steelmaking plant as an upper layer and a programmable logic controller interposed between the process computer and various kinds of equipment of the steelmaking plant and a DCS being an instrumentation controller as a lower layer. The programmable logic controller controls actuators of the manufacturing apparatus and acquires sensor information from sensors for performing process measurement of the temperature, pressure, speed and so on of manufacturing process according to the output from the process computer, so as to mainly take charge of real-time control at a high speed in a period of about several milliseconds to several hundreds of milliseconds.
Patent Document 1: Japanese Laid-open Patent Publication No. 2000-137662
Besides, the programmable logic controller has the reliability and stability secured for the operation but has a problem of a low development efficiency of software for complicated or large-scale arithmetic processing accompanied by convergent arithmetic and learning arithmetic. On the other hand, the arithmetic processing ability of the general-purpose personal computer and the ease of developing software are drastically improved due to the development of the recent computer technology. From the viewpoints, there are many advantages in that the general-purpose personal computer is introduced into the same layer on the plant control system as that of the programmable logic controller to take charge of the arithmetic processing.
However, the general-purpose personal computer is superior in the arithmetic processing ability and the ease of developing software, but is inferior in reliability of punctuality and so on in the industrial application. Therefore, the general-purpose personal computer is in danger of causing a trouble when it is just connected to the various kinds of equipment of the steelmaking plant and controls them.
Relating to this kind of technology, Patent Document 1 discloses that the programmable logic controller is not directly connected to the host computer but is connected to the host computer via a display device having an arithmetic processing ability and a data communication suitability higher than those of the programmable logic controller. In Patent Document 1, the display device performs protocol conversion so that programmable logic controllers can be controlled by a common program on the host computer side.